iridium

In rats, 20-nano-meter–diameter agglomerations of at least 100 radioactively labeled iridium particles entered the brain whether inhaled through the nose or pumped directly into the lungs.

At locations around the globe, geologists had found an unusually high concentration of iridium in the layer of sedimentary rocks that was formed about 65 million years ago.

Iridium is an element that is usually uncommon near Earth’s surface, but it is abundant in some meteorites.

Therefore, Alvarez and his colleagues concluded that it was likely that the iridium in sedimentary rocks deposited at the K-T boundary had originated in a giant meteorite or asteroid.

One interesting test of the Alvarez hypothesis is based on the presence of the rare-earth element iridium (Ir).

Debris thrown into the atmosphere by an asteroid collision would presumably contain large amounts of iridium, and atmospheric currents would carry this material all over the globe.

A search of sedimentary deposits that span the boundary between the Cretaceous and Tertiary periods shows that there is a dramatic increase in the abundance of iridium briefly and precisely at this boundary.

This iridium anomaly offers strong support for the Alvarez hypothesis even though no asteroid itself has ever been recovered.

Scientists felt that they could get an idea of how long the extinctions took by determining how long it took to deposit this one centimeter of clay and they thought they could determine the time it took to deposit the clay by determining the amount of the element iridium (Ir) it contained.